Cathode-ray oscilloscope circuit



Patented Nov. 2, 1948 CATHODE-RAY OSCILLOSCOPE CIRCUIT George Hecht, Astoria, N. Y., and Lamed A.

Meacham, Summit, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N..Y., a corporation of New York Application Api-1125, 1947, serial No. 743,980 claims.` (c1. 315-22) This invention relates to an improvement in cathode ray tube circuits, particularly useful where it is desired to establish a bright vertical reference line horizontally centered on the screen of the oscilloscope, the trace of the electron beam being elsewhere blanked inthe absence of an impulse corresponding, say, to the echo of a radar beam from an observed object. T-he invention may be termed a coincidence circuit, for the reason that the bright vertical line appears when the horizontal deflecting voltage, or current, passes through zero.

It lscommon in such circuits to use a tube having vertical and horizontal pairs of electrostatic deflection plates and to. arrange lthat a deflecting voltage on the horizontal plates be synchronized with the left and right pointing of a radar beam searching for a distant object, while a rapidly repetitive sweep voltage is applied to the vertical plates. It is arranged that an echo received from the object shall create a brightening voltage on the intensity grid of the cathode ray tube, producing a bright spot on the screen in a position corresponding vertically to .the range and horizontally to the azimuth of the reiiecting object from a reference direction, thercentral pointing of the exploring beam. An azimuth y voltage sweeps, as the radar'beam moves left and right, between positive and negative values, passing through zero when the beam passes through the reference direction. To define this direction horizontally, in the present invention the horizontal plates are maintained at the same potential in the absence of a deecting voltage.

By known means the azimuth voltage, whatever its polarity, is enabled to change the potentials of the horizontal plates equally and oppositely by amounts proportional to the instantaneous azimuth of the beam, whereby the mean potential of the plates is kept constant but the lelectron stream is deflected to left or right of the vertical center line of the oscilloscope screen as the radar beam itself is moved. If to the intensity grid of the oscilloscope a potential is applied of suitable magnitude to permit the electron stream to illuminate the screen, .the entire screen would be bright, because of the recurrent vertical and horizontal sweeps. Provision is therefore made for decreasing the intensity grid voltage to suppress the electron stream until a received echo momentarily unblanks the trace.

To refer the 4echo spot to the vertical horizontally centered line of the screen, this line must be made continuously bright, suppressing the trace either side of center. This is accomplished by the circuit of the present invention, which so controls the voltage on the intensity grid that the trace is blanked except at the center of the horizontal sweep. Brightening voltage pulses corresponding to radar echoes are applied to the intensity grid by means unconnected with the present invention.

It is therefore the general object of ythe invention to facilitate the location in azimuth of an object detected by a searching radar beam and represented by a bright spot on the screen of a cathode ray oscilloscope. Particularly, an object of the invention is to provide means for establishing a bright vertical line on the screen in a central position corresponding to the passage through zero of a voltage which varies with the pointing of'the radar beam and is zero when tha pointing is -in the reference direction. I

When the objects searched for are of considerable size, it is allowable to have the vertical bright line of width convenient for observation Without risk of obscuring the echo-spot, if this coincides with the central position. ,The circuit of the invention includes means for varying as desired the width of the reference line, and this is another object of the invention.

The invention will be understood from the fol-v I lowing description, referring to the accompanying drawings in which:

Fig. 1 shows the circuit of the invention in a preferred embodiment;

Fig. 2 shows curves relating the voltages on the horizontal plates of the oscilloscope to the azimuth voltage; and

Fig. 3 is a curve showing the relation of the azimuth voltage to the voltage on the intensity grid of the oscilloscope.

Referring to Fig. 1, II) indicates symbolically a source of voltage varying, in the example chosen, symmetrically between -6 and +6 volts, corresponding respectively to thev extreme left and right facings of a radar beam. Since the production of such a beam and the circuit of which the output is the azimuth voltage are no part of the present invention, the apparatus therein concerned is not shown in Fig. 1.

The voltage from source I0 may be applied through the resistor I2 to the control grid of tube Il. This control grid is also connected through resistor I5 to .the anode of the same tube, and through the variable resistor I6 to -200 volts on conductor I'I. The anode of tube Il is also connected through resistor I3 land conductor I I to a source of +450 volts, and by conductor I8 to horizontal plate HI of cathode ray oscilloscope i9. Variable resistor I6 is normally adjusted so that when zero voltage is supplied from source I0, the anode of tube lirests at +225 volts with respect to ground, the grid then having a bias oi approximately -6 volts, produced by IR drop in resistor i2. Oscilloscope I9 is a conventional cathode ray tube, understood to be provided with the usual elements, oi which it is necessary here to show only screen 20 with vertical line 2l horizontally centered, cathode 22, intensity grid 23 and deecting plates Hl and H2 for the horizontal, Vi and V2 for the vertical deflection. The electron stream is understood to be normally biased, by means not shown, to the bottom of screen 20; a repetitive saw-tooth voltage 24 is applied between vercal plates Vi and V2 causing the electron trace on screen. 20, it not blanked, to traverse the screen irombottom to 'top at a repetitive rate much greater than the rate of horizontal sweep which is to be followed by a deecting voltage on plates Hi and H2. The

. period of saw-tooth voltage 24 is the interval between successive radar pulses, which may oc- 4 v l tion changes in a reverse sense the potentials of conductors I8 and 35, connected to their respective anodes. In a horizontal sweep cyclethe input potential varies from -6 to +6 .volts and the voltages on plates HI- and H2 vary from about V+392 volts to about +58 volts, and from +58 volts to +392 volts. respectively, or vice versa, de-

-on conductor 4B.

pending on the polarity of the voltage from source I0,`their average value remaining +225 volts. 'I'his voltage variation of plates HI and H2 is shown in Fig. 2.

There is now provided a normal voltage of mon resistance 49 being included between -200 cur thousands oi times a second while the horlvolts and the ends of DI and D2 remote from their respective voltage supplies, resistance 49 being variable from zero to 50,000 ohms by adjustment of tap 50. With zero voltage from source l0, plates HI and H2 are each at +225 volts potential and the potential of intensity grid l23 is +200 volts, suilicient to brighten the electron trace on screen 20. At the same time each of inverter circuit is employed which includes tube Il o1 which the anode is connected to +450 volts through resistor 26 and conductor I4, and through resistors 28 and 29 to the anode of tube li. From the junction of those resistors is taken a connection through resistor 30 to the control grid of tube Il and through resistor 3l to -200 volts on conductor I1. The screen grids of both tubes Il and Il are connected throughindividual 180- ohm resistors through resistor 34 to +200 volts, the junction with resistor 34 being capacitatively by-passed to ground. As a result both screen grids are at a'potential approximately 150 volts positive to ground. Tubes Il and Il" are suitably each a 6AG7; cathode heating power is understood, but not shown, for these tubes and for the tubes later to be mentioned.

The values of resistances 26, 28, 29 and 3 l given in a later table,.are so proportioned that in the presence of zero voltage from source l0, the anodes of tubes ll and Il' are each 225 volts positive to ground, their grids each being approximately 6 volts negative to ground, whereas any deecting voltage from source l0 other than zero causes the anodes of tubes l l and Il to depart from +225 volts by equal and opposite amounts.

It will be recognized that the circuit of tubes Il and Il' is adapted, with modicatlons, from that disclosed by R. E. H. Carpenter in United States Patent 1,999,318, April 30, 1935.

The anode of tube il' is connected via conductor 35 to the horizontal plate H2 of oscilloscope I9; thus for zero deflecting voltage there is no potential dl'erence between plates HI and H2. and the electron stream, unless blanked by a voltage on the intensity grid of the oscilloscope.

will. produce a bright spot horizontally centered tubes 4B and 41 fis cut off, their cathodes being volts positive while each o! their grids is 87 volts positive when tap 50 is set to short all of resistance 49. In both DI and D2, 0.46 megohm is connected between conductor I8 (or 35) and the corresponding grid; 1.0 megohms between this grid and resistor 49.

Now, when a positive voltage is received from source l0 by the grid of tube-I l, the anode voltage of that tube falls while the like voltage of tube il' rises, and with these anode voltages plates Hl and H2 are no longer at the same potential and the electron stream moves toward plate H2.

At the same time, the increased voltage of con-v ductor 35 raises the grid voltage of tube 41 above cut-o5; the anode voltage vof this tube consequently falls taking with 4it the anode voltage of tube 46 and the voltage of grid 23. The electron stream is therewith blanked. The grid voltage drop at tube d6 has here n'o eect.

The voltage drop at the anode ofA tubes i8 and 41 will similarly appear if the voltage from source i0 is negative,'being now occasioned by the voltage increase on conductor i8 which carries the grid voltage of tube above cut-oli.

How far the voltage from source ill must de- -part in either direction from zero in order to reduce the voltage element on grid 23 enough to blank the trace depends on the initial bias of the grids of tubes 46 and 41; the further these tubes are initially biased below cut-oi, the more must be the voltage change from zero at source l0, and so the wider the 1ine.2i.. This line may appear as of finite width because many repetitions of the vertical sweep voltage 24 take place in the interval each side of zero voltage from source i0, and the corresponding electron spot traces fuse into a vertical rectangle centered horizontally between plates Hi and H2.

Line 2l is widest when tap 50 shorts the Whole of resistor 49. .Moving tap 50 downward obviously increases the initial positive voltage on the grids of tubes 46 and 41, bringing these tubes nearer to plate current conduction, so that a smaller change in the .voltage on conductors I@ and il is needed to make conducting one or the other of tubes 4B, Il, therewith lowering to the blanking value the voltage on grid 2l.

Fig. 3 shows the variation in Avoltage on grid 3.3 as the voltage of source Ill sweeps from positive to negative extreme. values infollowing the position of the radar beam. f -It-is easy to calibrate the voltage from source I ijn'terms of horizontal angle from dead ahead, or in feet, if desired. An echo from an irradiated object may, by known means, be caused to add an independent brightening voltage pulse to intensity grid 2 3; in such case a spot, as at 5l, appears on screen 20. The source of the independent brightening-voltage is symbolically shown as radar receiver 00 producing, when an echo is received, a sharp positive voltage pulse E applied to grid Z3 by way of conductor 85; the electron beam is thereby momentarily unblanked and spot El appears. Therange of the object is shown by the vertical location of spot M above a horizontal line (not indicated) on the screen at the foot of line 2|, and the bearlng of the object is shown by the horizontal displacement of spot Il left or right of line 2l.

If some value other than +200 volts is desired for the normal bright potential on grid 23, acapacitor may obviously be connected in series with this grid, and a grid leak connected from the grid to a suitable bias potential, in conventional manner.

While there is shown in Fig. 1 an oscilloscope with electrostatic deilecting plates, itis, of course, possible to provide magnetic deiiecting coils for both vertical and horizontal deflections. The vertical deflecting coil is to be connected in the conventional manner, while the horizontal deflecting coil is connected at its mld-point to ground and at its terminals to one and to the other of conductors I8 and 35, through suitable resistors or other coupling means. Equal and opposing magnetic iields are produced by the currents in the two halves of the horizontal coil when the potentials are the same on conductors I8 and 35, and these iields are appropriately unbalanced when the potentials are opposltely varied by a voltage from source I 0 other than zero. It is within the skill of the art to make the necessary allowance for the inductances of the coils.

Other changes from the circuit shown in Fig. l will suggest themselves to those skilled in the art, who will recognize that the invention consists in providing a negative voltage pulse on grid 23. when the vpotential of either of conduc- Resistor Resistance 40,000 ohms. 20,000 ohms.

me ohnii.

mego m approximate setting). 20,000 ohms. 1.20 megohm. l 25 m 1.80 ohms. 0.515 megohm.

,800 ohms.

s whichA varies in sign and value with the voltage from source I0; the voltage oi the control grid is normally great enough to permit the appearance of spot 5I on screen 20 and is reduced below thisvalue whenever the positioning voltage departs from'zero in either direction by a pre- The reader wm recognize that in the case of' selected and controllable amount.4 Obviously, by a like circuit, the vertical appearance of the spot couldbe conned to a position vertically centered on the screen, in the absence of a brightening voltage corresponding to an observed object.

What is claimed is:

1. A'cathode-ray oscilloscope circuit responsive to a source of voltage varying in each direction from zero, comprising a cathode-ray oscilloscope provided at least with a screen, a source of cathode-rays adapted to produce on the screen a bright spot, a control grid of which the voltage determines the appearance of the spot and a pair of horizontal deilecting plates, a phase inverter circuit controlled by the varying voltage to applyl to the plates separately voltages varying in proportion to the varying voltage equally and oppositely from a normal value, means for applying to the control grid a voltage normally adequate to insure the appearance of the spot and means controlled by the inverter circuit to reduce the control grid voltage when either of the plate voltages departs positively by a prescribed amount from the normal value.

2. For a cathode-ray oscilloscope provided at least with a screen, a source of cathode-rays adapted to produce on the screen a bright spot, a control grid of which the voltage determines the appearance of the spot and with means responsive to an applied voltage for deiiecting the spot, means for defining the spot position corresponding to zero voltage applied to the deliecting means comprising means for applying to the control grid a voltage normally adequate to insure the appearance of the spot, means for applying to the deecting means a variable spot positioning voltage and means controlled by the .lastnamed means for reducing the control grid voltage when the positioning voltage varies in either direction by a prescribed amount from zero.

3. In a system of apparatus comprising a cathode-ray oscilloscope provided at least with a screen, a source of cathode-rays adapted to pro- A duce on the screen a bright spot, a control grid of which the voltage determines the appearance of the spot and with horizontal and vertical spot deflecting means, a source of repetitive voltage applied to the vertical deecting means and a source of representative voltage in accordance with the value and sign of which the spot is horizontally positioned on the screen, means for dening a vertical bright line on the screen horizontally corresponding to the zero of the representative voltage, comprising a phase inverter circuit controlled by the representative voltage and applying to the horizontal delecting means a net voltage thereacross normally zero and varying from zero proportionally to the value and in accordancewith .the sign of the representative voltage, means for applying to the control grid a voltage normally adequate to insure the appearance of the spot and means controlled by the Vinverter circuit for reducing the control grid voltage when the n et voltage departs in either direction from zero by a preselected amount.

4. A system of apparatus as in claim 3, including means for' varying the preselected amount, therebyl controlling the width of the vertical bright line.

acsaess 5. In a cathode-ray oscilloscope provided at least with a screen. a source of cathode-rays adapted to produce on the screen a bright spot. a

control grid of which the voltage determines the appearance of the spot and with means responsive to an applied voltage for deecting the spot, means for confining the appearance of the spot to a portion o the screen the center of which corresponds to zerovoltage applied to the deflect ing means comprising means for applying to the control grid Va voltage normally adequate to insure the appearance of the spot, means for applying to the deecting means a variable spot positioning voltage, means controlled by the lastnamed means for reducing the control grid voltage when the positioning voltage varies in either direction from zero by a controllable amount cor'- responding to the desired extentof apce of the spot.

The following references are of record in the file of this patent:

STATES PATENTS July 1, 1947 

